Protein-protein interactions underlying molecular recognition are studied, utilizing antibodies (mAbs) specific for the protein hen egg white lysozyme (HEL), a protein which has long served as a prototype for investigating the specificity of immune recognition. In order to understand the molecular basis of interaction between an antibody and a protein antigen, the details of three structures must be defined: the antigenic epitope, the complementary paratope, and the interface of the antibody-protein complex. Mapping the epitopes recognized by a series of mAbs specific for HEL has allowed partial definition of the antigenic structure of this protein. Our results indicate that antigenic structure of HEL, as recognized by the immune system, is an emergent property which changes during the course of the immune response. The X-ray structure of the HyHEL-26 Fab-HEL complex has been solved and is being refined, bringing total structures of mAbs from our laboratory to 3. The HyHEL-26 epitope is nearly identical to that recognized by HyHEL-10, and the primary and genetic structures of HyHEL-26 are closely related to those of HyHEL-8 and -10. These results represent the first time structures have been obtained for 2 structurally related antibodies complexed to the same protein epitope, and we now have an unprecedented system for studying structure-function relationships. We have begun to experimentally investigate structure-function relationships in the interface by site-specific mutagenesis of both antigen and antibodies. These studies are beginning to define fundamental principles that will allow prediction of function from structure, principles that are critical to such applications as antibody design and vaccine development. We are also approaching the problem of vaccine development by investigating immunogenicity and protective epitopes in Shigella flexii. In addition, we have begun investigations which suggest a significant influence of antigenic exposure on development of the specificity repertoire and plasmacytomagenesis.